1. Field of the Invention
The present invention relates to a tire vulcanization mold for manufacturing a tire, more particularly to a structure of an improved air vent passage to reduce spew formation on an outer surface of a tire while permitting the escape of gas from the mold.
2. Description of the Related Art
Typically, a tire is produced through a vulcanizing process that molds and vulcanizes a green tire using a tire vulcanization mold with an inner surface for molding a tire. In the vulcanizing process, the green tire is heated and pressed onto the inner surface of the mold. In order to precisely mold a vulcanized tire, the green tire should be fitted closely and uniformly on the inner surface of the mold during the vulcanizing process.
However, in the event that whole portions of the green tire do not contact with the inner surface of the mold simultaneously, air would be trapped therebetween and remain. Such air may not only prevent smooth rubber flow during vulcanization process, but also causes molding defects such as a dent on the outer surface of the tire.
As shown in FIG. 9, in order to prevent molding defects of a tire, a tire vulcanization mold (d) having air vent passages (c) extending from an inner surface (a) to an exterior of the mold for the escape of gas is proposed. The tire vulcanization mold is provided with a bladder (g) which is inflated in a cavity of the green tire placed in the mold so as to press the green tire in the normal direction (b) to the inner surface (d) of the mold. The vent passages (c) also extend in substantially parallel with the normal direction (b) to the inner surface (d) of the mold.
Thus, air as well as a part of plasticized rubber (i) pressed by the bladder (g) goes into the vent passage (c) easily during vulcanizing process. After vulcanizing process, rubber insertion in the vent passage (c) turns to a spew remained on the outer surface of the tire. The tire having spews is liable to decrease in quality and appearance thereof. In order to prevent spew formation on outer surface of tires, the vent passage having a small inside diameter in which rubber is difficult to enter is proposed. However, such a vent passage may not work for the escape of gas sufficiently.